Source waveforms for electroseismic exploration

ABSTRACT

A method for seismic exploration using conversions between electromagnetic and seismic energy, with particular attention to the electromagnetic source waveform used. According to the invention, source waveforms are correlated with reference waveforms selected to minimize correlation side lobes. Line power at 60 Hz may be used to provide a waveform element which may be sequenced by a binary code to generate an extended source waveform segment with minimal correlation side lobes. Preferred binary codes include Golay complementary pairs and maximal length shift-register sequences.

This application claims the benefit of U.S. Provisional Application No.60/191,041 filed on Mar. 21, 2000.

FIELD OF THE INVENTION

This invention relates to the field of geophysical prospecting. Moreparticularly, the invention pertains to source waveforms for use inelectroseismic exploration.

BACKGROUND OF THE INVENTION

The electroseismic method is a geophysical prospecting tool aimed atcreating images of subsurface formations using conversions betweenelectromagnetic and seismic energy. The electroseismic method isdescribed in U.S. Pat. No. 5,877,995 (Thompson. et al.). The essence ofthe electroseismic method is that high levels of electrical energy aretransmitted into the ground at or near the surface, and the electricalenergy is converted to seismic energy by the interaction of undergroundfluids, including hydrocarbons, with the rock matrix. The seismic wavesare detected at or near the surface by seismic receivers. To beeffective, this method requires an input current source with thefollowing characteristics:

-   -   The source should produce large current levels over extended        time.    -   The source should have high electrical efficiency.    -   The source should contain little or no DC to avoid plating the        electrode array.    -   The frequency content of the source should be adequate for the        exploration needs.    -   The correlation of the source waveform with its reference should        have sufficiently low side lobe levels.

Little has been published to date on electroseismic waveforms because ofthe newness of the technique. However, in conventional seismicexploration, a seismic vibrator is sometimes used as an energy source togenerate a controlled wavetrain (known as a sweep) which is injectedinto the earth. When the resulting recorded seismic data are correlatedwith the sweep wavetrain or other reference, the correlated recordresembles a conventional seismic record such as that which results froman impulsive source.

When a source waveform is correlated with its associated reference,there will typically be a large peak at the onset time of the waveformsurrounded by lower peaks at earlier and later times. These lower peaksare the correlation side lobes. Correlation side lobes are undesirablebecause they can mask smaller desired seismic returns.

It should be noted that the source waveform is just one piece of theelectroseismic system. Other factors of importance include the powerwaveform synthesizer (that creates the waveform) as well as the inputelectrode array, the seismic receiver arrays and various fieldimplementation issues.

As stated above, there is little current technology on electroseismicwaveforms because of the newness of the technique. Some obviousapproaches might include pulsing or pseudo-random square-wave sequences.Repeated pulses are inefficient (in energy/second) compared tocontinuous waveforms. Square-wave waveforms would be expensive toimplement at the required high current levels and would also dissipateenergy in unwanted high-frequency components.

What is needed is a source waveform that satisfies the requirementsstated above. The present invention satisfies this need.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is a method for electroseismicprospecting comprising the steps of (a) selecting a source waveform andcorresponding reference waveform, both chosen to reduce correlation sidelobe amplitudes, (b) generating an electrical signal based on the sourcewaveform, (c) transmitting the electrical signal into the ground, (d)detecting and recording the seismic signals resulting from conversion ofthe electrical signal to seismic energy in subterranean formations, and(e) correlating the resulting seismic signals with a reference waveformto produce a correlated seismic record. Preferably, the referencewaveform is chosen to substantially minimize side lobes when correlatedwith the particular source waveform used.

The source waveform may be constructed from individual cycles of a 60cycles/sec (Hz) sine wave, i.e., standard AC electrical power, with thepolarity of some such cycles inverted as governed by a binary sequencecode. The binary code is selected to custom design an extended, butfinite, source wave that has a reference wave that substantiallyminimizes correlation side lobes when the source wave and the referencewave are correlated together. The reference wave may be the source waveitself or a waveform derived from the source wave. Where deeperpenetration of the subsurface is desired, another embodiment of thepresent invention constructs frequencies lower than 60 Hz by switchingbetween the three phases of a 3-phase power source.

In some embodiments of the invention, the binary sequence used is amaximal length shift-register sequence, and circular correlation(defined below) is used for the last step. In other embodiments, twosource waves are transmitted into the ground. One is a 60 Hz sinusoidwave element sequenced by one member of a Golay complementary sequencepair; the other is the same wave element sequenced by the other Golaypair member. The resulting seismic returns are separately correlatedwith their respective input waves and then summed. Both the Golaysequences and the maximal length shift-register sequence have excellentcorrelation side lobe reduction properties, with the side lobes beingtheoretically reduced to zero in the case of the Golay sequences.

According to the present invention, side lobes may be further reduced,for any pseudo-random binary sequence-generated source waveform, bymaking the source wave as long as possible before recording equipmentlimitations require that the source generation must be interrupted.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will be better understood byreferring to the following detailed description and the attacheddrawings in which:

FIG. 1A illustrates a 60 Hz waveform element, and

FIG. 1B illustrates the autocorrelation of such waveform element.

FIG. 1C shows a waveform element of frequency less than 60 Hz,constructed from three-phase 60 Hz waves.

FIG. 1D shows the autocorrelation of the waveform element of FIG. 1C.

FIG. 2A and FIG. 2B show a Golay complementary sequence pair with a 60Hz waveform element.

FIG. 3A and FIG. 3B show the autocorrelation of the waves in FIG. 2A andFIG. 2B, respectively.

FIG. 3C illustrates the sum of these autocorrelations.

FIG. 4 is a schematic diagram of a shift register.

FIG. 5A illustrates a maximal length shift-register sequence with 60 Hzwaveform element, and FIG. 5B shows its autocorrelation.

FIG. 6A illustrates a modified shift-register sequence with 60 Hzwaveform element, and FIG. 6B shows the cross correlation of the wave ofFIG. 6A with the wave of FIG. 5A.

FIG. 7 illustrates a typical field setup for the present invention.

FIG. 8 illustrates test results.

The invention will be described in connection with its preferredembodiments. However, to the extent that the following detaileddescription is specific to a particular embodiment or a particular useof the invention, this is intended to be illustrative only, and is notto be construed as limiting the scope of the invention. On the contrary,it is intended to cover all alternatives, modifications and equivalentsthat may be included within the spirit and scope of the invention, asdefined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a method for determining source waveforms touse for electroseismic prospecting for oil and gas. The source signalstaught by the present invention in some of its embodiments are in theclass of binary-coded waveforms. A binary-coded waveform consists of asequence of elements. The individual elements might be, for example, asingle cycle of a 60 Hz sine wave. In fact, waveforms that are made upof segments of 60 Hz sinusoids (or whatever the local commercial powerfrequency may be) are particularly economical for the electroseismiccase because this source type can be formed using simple switching ofcommercial powerline signals. These waveform segments are piecedtogether with polarities specified by a binary sequence. Preferably, thebinary sequence is designed, as explained below, to give minimal sidelobes while the waveform element is designed to optimize the frequencycontent of the source.

The autocorrelation of a binary-coded waveform will give a main waveletthat is the autocorrelation of the individual waveform element. (See thediscussion below of Golay sequences for an example of this.) FIG. 1Ashows a single cycle 10 of a 60 Hz sinusoid. Its autocorrelation 20 isgiven in FIG. 1B. This is easily understood by referring to, forexample, Seismic Data Processing, Ozdogan Yilmaz, Society of ExplorationGeophysicists, 1987, 18-19. This waveform element 10 is probablyadequate for relatively shallow targets. Different elements with lowerfrequencies can be constructed when three-phase power is available. Anexample 30 is given in FIG. 1C with its associated autocorrelation 40 inFIG. 1D.

By way of further explanation of FIG. 1C, three-phase power provides sixsinusoids with 60 degrees of phase shift between them. A variety ofapproximately square-wave signals can be constructed by switching fromone sinusoid to the next at the crossover point. As one signal starts tofall off from its peak, the output is switched to the next signal whichis rising to its peak. In this manner, an approximate square wave can beconstructed. The square wave can be made with a desired width that hasan integer number of such cycle switches, and hence with a correspondingfrequency less than 60 Hz. FIG. 1C illustrates an example where the peakis prolonged by switching five times to the next-in-phase sinusoid toyield a square wave with frequency of about 20 Hz.

The construction of the waveform element is an important aspect of thedesign of the electroseismic source. Methods such as genetic algorithmscan be used to determine a desirable element for a given target withspecified seismic attenuation and electromagnetic skin depth. Ingeneral, the deeper the target, the lower the preferred frequencybecause higher frequencies tend to be absorbed, reducing efficiency.Fortuitously, 60 Hz gives good results for targets between approximately100 and 500 feet in depth, for typical sediments. Furthermore, the 60 Hzwave element, although not preferred, may be used successfully to muchgreater depths, on the order of 5,000 feet. The practical andconvenience advantages of constructing the waveform from 60 Hz linepower will be obvious to those trained in the seismic art. Furthermore,such hardware implementation is easier for a single frequency sinusoidwave element, in part because efficiency does not have to be sacrificedfor broadband amplification.

Correlation side lobes are of critical importance in electroseismicexploration because there can be a very large peak at zero time. Thislarge peak comes from unavoidable direct pickup at the receivers fromfields related to the input currents. The large peaks will havesignificant correlation side lobes. Even though these direct-pickup sidelobes are reduced from the peak amplitude, they may still be largeenough to mask the much smaller desired electroseismic returns. Thelevel of the direct pickup can be moderated by proper field designand/or by other innovations such as receiver modifications; nonetheless,it is best to minimize the impact of the direct pickup by using a sourcewaveform with minimal correlation side lobes. (Correlating with theappropriate reference waveform for the source waveform will reduce allside lobes, including the direct pickup side lobes because the directpickup also is caused by the applied signal.)

In the preceding discussion of correlating a source wave with areference wave to minimize side lobes, the source wave is a surrogatefor the electroseismic response. In the present invention, it is therecorded seismic response to the electrical source wave that iscorrelated, in a later processing step, with the chosen reference wave.The present invention is based on detecting the linear component of theelectroseismic response. As a consequence of linearity, such responsewill be proportional to the source wave. Therefore, custom-designing asource wave and a reference wave to have a large central peak andminimal side lobes when correlated together insures that the seismicresponse to that same source wave will similarly produce a large centralpeak and minimal side lobes when correlated with that same referencewave.

There are at least two types of binary sequences that according to thepresent invention are well suited for electroseismic waveforms. One isthe maximal length shift-register sequence which is found in Golomb, S.,Digital Communications with Space Applications, Prentice Hall, Inc.(1964). The other is the Golay complementary sequence pair, which isfound in Golay, M. J. E., “Complementary Series”, IRE Transactions onInformation Theory (1961) Vol. 7, 82-87. These sequences can beconfigured to give substantially minimal correlation side lobes.

Golay Sequence

In one embodiment, the present invention uses sinusoidal wave elementssequenced by Golay complementary sequence pairs. A complimentary pair ofGolay series is defined by Golay as a pair of equally long, finitesequences of two kinds of elements which have the property that thenumber of pairs of like elements with any one given separation in oneseries (viewing each series as cyclical for purposes of determiningseparation) is equal to the number of pairs of unlike elements with thesame given separation in the other series. These sequence pairs have theproperty that the sums of the sequence autocorrelations have zero sidelobes. One such pair of length 8 is {−1 −1 −1 1 −1 −1 1 −1} and {−1 −1−1 1 1 1 −1 1}. This pair of series can be seen to satisfy thedefinition given above. For example, taking a separation of 3 spaces,the first series has 6 like pairs and 2 unlike pairs while the secondseries has 2 like pairs and 6 unlike, and so on. This pair of series oflength 8 is shown in FIGS. 2A and 2B with a 60 Hz element. FIG. 2A showsthe first-mentioned sequence 50; FIG. 2B shows the second mentionedsequence 60. A “1” in either sequence means a single cycle of 60 Hzsinusoid with normal polarity, illustrated by the cycle starting at 52and ending at 54 in FIG. 2A; a “−1” means a 60 Hz wave element withinverted polarity, illustrated by the cycle starting at 54 and ending at56 in FIG. 2A.

The autocorrelations and sum are shown in FIGS. 3A-C. FIG. 3A shows theautocorrelation 70 of the extended waveform segment 50; autocorrelation80 of extended waveform segment 60 is shown in FIG. 3B. FIG. 3C depictsthe sum 90 of autocorrelations 70 and 80. In theory, the side lobescancel completely. The sequence-pair approach is also very efficient inthe sense that the waveform is continuous, i.e., without breaks ascontrasted with, for example, the modified shift-register sequencedescribed below. The drawback is that the side lobe cancellation istheoretical, depending as it does on proper subtraction of therelatively large side lobes of autocorrelations 70 and 80. This can beproblematic in practice where there can be frequency drifts in the powersupply or fluctuations in the signal amplitude.

Registering the sequence to a reference line frequency, scaling the datato match amplitudes, and selecting particular Golay sequence pairs thathave relatively low side lobes at the lags of interest can minimizethese problems.

For a typical field implementation, a preferred embodiment of thepresent invention might include a 60 Hz waveform element and a Golaysequence of length 1664 (27.7 seconds). Golay sequences can beconstructed of smaller sequences using methods discussed in Kounias, S.,Koukouvinos, C., and Sotirakoglou, K., “On Golay Sequences”, DiscreteMathematics, (1991) Elsevier Science Publishers B. V., Vol. 92, 177-185.Golay sequences exist only in certain lengths, e.g., there are Golaysequences of length 8 but none of length 6. Golay showed that sequencesexist for lengths given by 2^(j) 10^(k) 26^(l), where j, k, and l arenonnegative integers, i.e., 0, 1, 2 . . . (see Kounias, p. 178). Otherlengths that can, but not necessarily do, have Golay sequence pairs aregiven by a²+b² where a and b are integers (including zero). For lengthsup to 50, Golay claims sequence pairs exist for the following lengths;2, 4, 8, 10, 16, 18, 20, 26, 32, 34, 36, 40 and 50 (see Golay, pg. 84).Kounias provides an algorithm for generating solutions for a givenlength (see Kounias, pg. 184). Lemma 1 in the Kounias paper (pg. 178)shows how to construct longer Golay sequences from shorter sequences. AGolay sequence pair of length nm can be constructed from a Golay pair oflength n and another Golay pair of length m. Following theseconstruction rules, there are many possible sequence pairs of aspecified length.

A particular pair for a given length can then be selected using, forexample, an exhaustive search to select a pair with minimal side lobesprior to cancellation. This selection will minimize the residual sidelobe energy from imperfect cancellation. Each of the Golay-pairsequences is run in the field as a separate source with some dead timeallotted after each to collect seismic returns. The referencecorrelations and summing are processing steps.

As an example of how to generate other complementary Golay series of agiven length, consider the pair of length 8 given above:

-   -   −1 −1 −1 1 −1 −1 1 −1 and −1 −1 −1 1 1 1 −1 1        Golay showed that any of the following operations on a given        pair produces another pair of complementary series:    -   (a) Interchanging the series;    -   (b) Reversing the first series;    -   (c) Reversing the second series;    -   (d) Altering (replacing each element by its opposite) the first        series;    -   (e) Altering the second series; or    -   (f) Altering the elements of even order in each series.        Following these rules, six more complementary series of length 8        are:    -   −1 −1 −1 1 1 1 −1 1 and −1 −1 −1 1 −1 −1 1 −1    -   −1 1 −1 −1 1 −1 −1 −1and −1 −1 −1 1 1 1 −1 1    -   −1 −1 −1 1 −1 −1 1 −1 and 1 −1 1 1 1 −1 −1 −1    -   1 1 1 −1 1 1 −1 1 and −1 −1 −1 1 1 1 −1 1    -   −1 −1 −1 1 −1 −1 1 −1 and 1 1 1 −1 −1 −1 1 −1    -   −1 1 −1 −1 −1 1 1 1 and −1 1 1 −1 −1 1 −1 −1 −1        Many more complementary series of length 8 (some of which may be        identical) can be generated by performing two or more of the six        operations given above. In addition, these operations can be        performed on the sub-sequences (e.g., pairs of length 4 and 2)        prior to combination to make a length 8 sequence.        Maximal Length Shift-Register Sequence

Golomb defines a pseudo-random binary sequence (“PRBS”) as any binarysequence generated by a deterministic process (such as a shift register)in such a way that the sequence will satisfy whatever tests forrandomness that may be selected (see Golomb, pp. 7-16). A shift registerof degree n is a device consisting of n consecutive binary (1, −1 or 1,0) storage positions or “registers”, which shifts the contents of eachregister to the next register down the line, in time to the regular beatof a clock or other timing device. In order to prevent the shiftregister from emptying by the end of n clock pulses, a “feedback term”may be compiled as a logical (i.e., Boolean) function of the contents ofthe n positions and fed back into the first position of the shiftregister.

For example, consider the case where n=4 and the feedback function is toadd the contents of the third and fourth registers, the sum to becomewhat is put into register 1 after the next shift empties it. Suchaddition of binary numbers is called module 2 addition and is denoted bythe symbol ⊕.

Thus in the binary {1,0} domain, 0⊕0=0; 0⊕1=1⊕0=1; and 1⊕1=0. Such ashift register is illustrated in FIG. 4.

It can be shown that this feedback function can be expressed as thefollowing recursion formula:X_(i)=X_(i-3)⊕X_(i-4)where X_(i) is the contents of any one of the four registers for thei-th shift. Thus, the contents of any register are the modulo 2 sum ofwhat was in that same register three shifts previously and what was inthat same register four shifts previously.

Starting the process with the contents of all four registers set to 1,i.e., X₀(X₁)=X₀(R₂)=X₀(R₃)=X₀(X₄)=1, the four registers take on thefollowing values before the numbers begin repeating:

i X_(i)(R₁) X_(i)(R₂) X_(i)(R₃) X_(i)(R₄) 0 1 1 1 1 1 0 1 1 1 2 0 0 1 13 0 0 0 1 4 1 0 0 0 5 0 1 0 0 6 0 0 1 0 7 1 0 0 1 8 1 1 0 0 9 0 1 1 0 101 0 1 1 11 0 1 0 1 12 1 0 1 0 13 1 1 0 1 14 1 1 1 0

The numbers generated in register 1 (the other registers generate thesame sequence with cyclic permutation) are the “shift-register sequence”for this particular shift register of length 4 and particular recursionrelationship. It can be seen that the number in register 1 for any valueof i is the modulo 2 sum of the numbers in register 3 and 4 one shiftpreviously which, in turn, are the same two numbers that were inregister 1 three and four shifts previously, as required by therecursion formula. This sequence satisfies, for example, the followingthree randomness tests which are proposed by Golomb at page 10 of hisbook:

-   -   1. In each period of the sequence, the number of ONE's differs        from the number of ZERO's by at most 1.    -   2. Among the “runs” of ONE's and of ZERO's in each period,        one-half the runs of each kind are of length one, one-fourth are        of length two, one-eighth are of length three, etc., as long as        these fractions give meaningful numbers of runs.    -   3. If a period of the sequence is compared, term by term, with        any cyclic shift of itself, the number of agreements differs        from the number of disagreements by at most 1.

In the sequence of length 15 generated above (register 1), there are 8ONE's and 7 ZERO's, satisfying test 1. The sequence has 5 runs of lengthone, 2 of length two, and 2 of length three, which closely satisfiestest 2. (A “run” occurs where a number is repeated in the sequence,except for a run of length one which is a single occurrence.) Comparingthe sequence of register 1 with that of register 4, which is a cyclicpermutation of the register 1 sequence, shifted 3 positions, one finds 7agreements and 8 disagreements, satisfying test 3. This consistentsatisfaction of randomness tests is a difference between pseudo-randomseries and truly random series. Because they are deterministic, each andevery pseudo-random series will satisfy the test. Truly random serieswill satisfy the tests only on average.

The output of any shift register is ultimately periodic, with a periodnot exceeding 2^(N) where n is the degree, or length, of the shiftregister (Golomb, p. 9). For linear recursion formulas, defined byGolomb at page 9, the period is at most 2^(n)−1. In the example above,where n=4, the period is 15 and therefore the sequence generated abovehas the maximum possible length, and accordingly is called a maximallength shift-register sequence.

An example of a maximal length shift-register sequence of length 7 is{−1 1 −1 1 1 1 −1}. FIG. 5A shows the resulting extended waveformsegment 100 using a 60 Hz element. The circular autocorrelation 110 ofwaveform 100 is shown in FIG. 5B. The central portion of waveform 110 isthe autocorrelation of a 60 Hz cycle and the side lobes are 60 Hz withrelative amplitude of 1/7 (for a length 7 sequence). This level of sidelobes might be acceptable for long sequences but alternate approachesare possible.

Foster and Sloan, for example, altered waveform 100 to include only thepositive binary elements 120, with the negative elements replaced byzero-amplitude elements, as shown in FIG. 6A. (Foster, M. R., and Sloan,R. W., “The Use of Pseudonoise Sequences to Code a Pulsed NeutronLogging Source”, Geophysics (1972) Vol. 37, 481-487). When circularlycorrelated using waveform 100 (FIG. 5A) as the reference, the result 130now has zero side lobes (FIG. 6B). The disadvantage of this approach isthat the peak value is roughly halved because of the zero-amplitudeelements.

To explain terms used above, autocorrelation means the correlation of asignal with itself. Circular correlation can be explained as follows: Ina standard correlation process, the signals are assumed to be“zero-padded” prior to correlation, i.e., the signal wave sequence andits reference are assumed to drop to zero amplitude before and after thesequence. The correlation process involves the cross product of onesignal and a shifted version of the second signal for various shifts.With the zero padding, the portion of the shifted signal that passes theend of the stationary signal has no effect because it is multiplied bythe appended zeros. In the case of circular correlation, the signals areassumed to repeat rather than have zero padding. Thus, as the shiftedsignal passes the end of the stationary signal in the correlationprocess, it begins to overlap the beginning of the stationary signal.Where circular correlation is used, it is used because it reduces theside lobes better than standard correlation.

The choice of a binary sequence and waveform element will depend on thetarget, the geology, and the field configuration. The decision on awaveform element would be based largely on the target depth and theexpected attenuation. The decision on the binary sequence type would bebased on the relative level of direct electromagnetic pickup and,consequently, the need for side lobe reduction. An appropriate selectionwill likely require computer modeling and field tests. As discussedfurther below, the longer the sequences, the more the side lobes will bereduced. Capabilities of the recording equipment are a practical limiton sequence length. Although the present invention is primarily for useon land, it can give useful results in a marine environment.

In general, longer binary sequences tend to produce lower correlationside lobes than shorter sequences. In the case of waveforms generated bymaximal length shift-register sequences, it is known that the amplitudesof the side lobes after the sequence is circularly correlated withitself varies as 1/L where L is the length of the sequence. Therefore,longer maximal length shift-register sequences are preferred. Trulyrandom sequences are known to exhibit a lesser reduction with length,varying inversely as the square root of the length, but this effect ismanifested on a statistical basis.

The other preferred type of binary coding for the present invention,Golay sequence pairs, are neither pseudo-random nor random. They alsotend to show an inverse dependence of side lobe amplitude withsequence-length, but the dependence is much weaker than 1/L. Althoughside lobe cancellation for Golay pairs is theoretically perfect,regardless of sequence-length, it is unlikely that this perfectcancellation will be achieved in practice as pointed out above.Therefore, Golay sequences are preferably chosen to have minimal sidelobes before the pairs are added together, and sequence length is onefactor in such a selection. Other than length, the basis for choosing asequence is the same as with PRBS sequences: one must try varioussequences and observe what side lobe attenuation is achieved.

The correlation for Golay sequences is standard correlation.Pseudo-random sequences need circular correlation to best reduce sidelobes, which tends to place a practical limit on the length of suchsequences. A pseudo-random sequence needs to be repeated (in the field)at least once to provide circular overlap. The first cycle can be usedonly to provide delayed correlation effects on the next cycle, and henceis lost for data-gathering purposes. While a longer PRBS is preferredfor side lobe reduction, a shorter PRBS is preferred to avoid data lossand waste of field time. The preferred compromise is to use anintermediate length PRBS and repeat it several times, perhaps three toseven cycles in all. Thus a maximal-length shift register sequence oflength 255 might be selected and repeated six times for a total of sevencycles. This would consume about 28 seconds in the field assuming a 60Hz wave element, and only 1/7 of the data would be, in effect, lost. Thedata thus gathered from such a PRBS example would compare fairly closelyin quantity to that obtained from a Golay-pair sequence of length 1664(as 6×255=1530 compares to 1664).

FIG. 7 is a diagram of a possible field layout for deployment of thepresent invention. Electric current is injected into the subsurface 140by applying a voltage from a power source 141 between two buriedelectrode wires 142 and 143. The electrode wires are typically bare 4/0copper cable. Other sizes of cable may be used as needed to conduct theneeded current. The electrode wires should be buried just deep enough tomake good electrical contact with the soil. Typically, this depth willbe in the range from 1 inch to 10 feet, but in some cases, such asparticularly dry surface soil, even deeper burial may be desirable.

FIG. 7 shows the current paths 144. The direction shown is that of theflow of electrons, from the negative electrode to the positiveelectrode. The power source 141 provides the current and consistsprimarily of a waveform generator capable of generating the binary-codedwaveform segments preferred by the present invention. The ultimate powersource is typically the local electrical utility power lines (connectionnot shown in FIG. 7). One or more generators may also be used. Thecurrent paths 144 shown in FIG. 7 represent those current paths thatpenetrate down to the depth of the target 145, typically a petroleumreservoir. Such current trajectories will be nearly vertical below theelectrode. Experience shows that the maximum vertical current 146 willtypically be directly under or even slightly to the outside of theelectrode wires. Accordingly, the maximum amplitude of the resultingsurface-directed seismic wave 147 (similar wave below positive electrodenot shown) will occur along the line of maximum vertical current, andthis determines the preferred location of the seismic detectors 148.

The seismic detectors may be placed anywhere on the surface, but thepreferred location is outside the electrode wires rather than betweenthe electrode wires. The seismic detectors may be geophones,hydrophones, accelerometers or any similar device. Such seismicequipment is well known to those skilled in the art. Preferably, theseismic detectors are buried beneath the surface to reduce seismicnoise.

Typically, the configuration shown in FIG. 7 is designed to cover theentire area of interest i.e., source and receivers will not need to berepeatedly moved to progressively cover the area of interest as withconventional seismic. This is one advantage of the electroseismicmethod, although a single setup is not essential to using the presentinvention. (The electrode wires and receivers may be dug up when theexperiment is over and moved to other locations.) Accordingly, thelength of the electrodes may vary between one-tenth of the reservoir(target) depth to several times the reservoir depth. The separationbetween electrodes in preferred embodiments of the present inventionwill be approximately equal to the target depth.

In electroseismic surveying, the electromagnetic source wave reaches allof the target at essentially the same instant of time. One is thereforeusually interested only in the upward-traveling seismic waves thatarrive at all of the geophones at approximately the same time, assumingthe geophones are deployed as in FIG. 7. Thus, in processing, theordinary seismic noise can be filtered out because it exhibits what inthe seismic art is called “moveout”. The source-to-receiver distance,called “offset”, is small in electroseismic prospecting. In aconventional seismic survey, a much larger surface area would have to becovered because the longer offsets would be needed from every shot pointto provide sufficient signal-to-noise ratio in common mid-point gathers.The area reduction in electroseismic vs. conventional seismic may beapproximately fourfold.

The binary-coded waveforms discussed above meet the five requirementsfor electroseismic exploration stated previously. The need for largecurrent levels is addressed since these are continuous waveforms insteadof, for example, pulses that would have significant dead time. The useof simple 60 Hz (or constructed 3-phase) elements also allow largecurrent levels and high electrical efficiency since the related hardwareis simplified. The lack of DC is assured since each waveform element(e.g., a full cycle at 60 Hz) has no DC component (i.e., its mean valueis zero); it follows, therefore, that a set of such elements would haveno DC component. The frequency content of the source can be matched tothe exploration target by adjusting the frequency of the waveformelement. Finally, minimization of side lobes has been discussed atlength.

To further explain the preceding statement about adjusting the frequencyof the source to achieve the desired depth penetration, neither thesource wave nor the return seismic response is composed of a singlefrequency. The phase inversions and (in some embodiments) the zeroing ofcertain elements produce waves composed of many frequencies in the senseof their Fourier analysis decomposition. This is necessary to theinvention, i.e., that the waves have a bandwidth of frequencies. If theseismic return wave were a single frequency, there would be no wave thatwould cross-correlate with it to produce a localized pulse in theprocessing step. A bandwidth of frequencies is needed to produce thedesired spike. Elementary Fourier analysis teaches that the sharper thespike, the wider the needed bandwidth. Thus, although the source andreturn waves each have a spread of frequencies, it is reasonable toexpect, and Fourier decomposition can prove, that the frequencydistribution of both waves will peak at the frequency of the buildingblock, the waveform element. Thus, the desired subsurface penetrationmay be achieved by varying the frequency of the waveform element.

FIG. 8 illustrates the results obtained by applying the presentinventive method at the Friendswood gas field in Texas. The regiondepicted is in the vicinity of Well No. 181. The site was selected basedon knowledge of gas leakage at the surface indicating shallow gasdeposits located above the producing formations. A conventional seismicsurvey of the area showed seismic bright reflections shown in FIG. 8, ofwhich band 160 is by far the most prominent. The five dark bodies 170represent regions of high amplitude electroseismic signal.Electroseismic signals of less than about half the peak value are madetransparent by the display. The detected signal was 10:1 or more abovebackground, on average. (FIG. 8 is an artist's rendition of a displayproduced by the GEOVIS program marketed by Geospace Corporation.)Vertical seismic profiles and previous seismic data were used toestablish time-to-depth conversion.

A test well was drilled to a depth of 1,000 feet to check the seismicinterpretations that can be made from FIG. 8 by anyone of ordinaryexperience in the seismic art. The well line is shown in FIG. 8 at 180.The well logs confirmed gas sands at four of the five locations 170. Theone not confirmed is the one intermediate in depth, which is barelycontacted by the well as can be seen in FIG. 8. The conventional seismicreflection surface 160 turned out to be a shale layer sealing theuppermost gas sand 170. (Shales can have much higher acoustic velocitiesthan surrounding substances which makes them strong seismic reflectors.)Only the uppermost of the five gas sands predicted by the presentinvention is predictable from the conventional seismic results. Notethat the present invention indicates in this example the hydrocarbondeposits themselves, not a structure that may or may not trap or includehydrocarbons.

The electroseismic source signal used at Friendswood was constructedfrom a 60 Hz sinusoid, using Golay complementary pair sequences oflength 1664, producing a sweep of duration 27.73 seconds. This sweep wasrepeated approximately 500 times for each of the Golay pair of signals.This repetition tends to reduce ambient noise, relative to the seismicsignal, because the ambient noise occurs at random phases relative tothe signal.

The field layout for the Friendswood test was similar to that shown inFIG. 7. The length of the electrode wires was approximately 800 feet andthe electrode spacing was approximately 650 feet. Geophones were placedat 180 surface locations on an 18×10 grid to the outside of one of theelectrodes only, this being sufficient to test the method. Two sets ofgeophone strings were used at each surface location. The geophones onone string differed from those on the other only in the direction of thecoil windings. The geophones used operate on the principle that slighttremors move a wire coil through a fixed magnetic field generating anelectric signal. Reversing the coil windings reverses the polarity ofthe unwanted electromagnetic pickup without affecting the desired signalgenerated by the moving coil. Combining the outputs of the twooppositely wound geophone strings tends to produce a cancellation ofunwanted pickup.

The signal generator, which may be called a power waveform synthesizer,used in the Friendswood test produced a power output of approximately100 kw, delivered at 120 volts peak voltage. Because the impedance ofthe ground is low, the waveform synthesizer must be capable of highcurrent levels. The primary challenge in designing or assembling such apower synthesizer is in meeting the high power (current) requirements.This can be done by persons skilled in hardware design usingcommercially available components.

Finally, the applied electrical signals were recorded in the field asthey were transmitted into the ground at Friendswood. This record isthen used as the correlation reference waveform in the data processingstage, thus providing the most accurate reference waveform possible, onethat accounts for actual line voltage and similar fluctuations. Therecorded signal may either be a voltage signal or a current signal. Inthe case of the test example represented by FIG. 8, a current signal wasrecorded.

The foregoing description is directed to particular embodiments of thepresent invention for the purpose of illustrating it. It will beapparent, however, to one skilled in the art that many modifications andvariations to the embodiments described herein are possible. Forexample, other source waveform elements and binary sequences can be usedas long as they satisfactorily meet the five requirements listed above.As noted previously, correlations side lobe amplitude varies inverselywith the length of the extended waveform segment for any pseudo-randomwaveform. Thus, there are many possible choices of waveform element andbinary sequencing that will give satisfactory results within theframework of the present invention as described above. Moreover, thepresent invention does not require that the source waveform be generatedby binary sequencing of a single waveform element, or by binarysequencing in any manner. All such modifications and variations areintended to be within the scope of the present invention, as defined inthe appended claims.

1. A method for electroseismic prospecting of a subterranean formation,said method comprising the steps of: (a) selecting a source waveform andcorresponding reference waveform, said two waveforms being selected toreduce amplitudes of side lobes produced by correlating said sourcewaveform with said reference waveform; (b) generating said sourcewaveform as an electrical signal and transmitting said electrical signalinto said subterranean formation; (c) detecting and recording seismicsignals resulting from conversion of said electrical signal to seismicenergy in said subterranean formation; and (d) correlating said recordedseismic signals with said reference waveform.to produce a correlatedseismic record; and (e) creating an image of the subterranean formationfrom the correlated seismic record.
 2. The method of claim 1, whereinsaid source waveform is constructed from a single element, said elementconsisting of a single full cycle of a preselected periodic waveform,said elements being pieced together with polarities sequentiallyspecified by a preselected binary code, said periodic waveform having afrequency predetermined to give desired depth penetration of saidsubterranean formation.
 3. The method of claim 2, wherein said waveformelement in a single cycle of a 60 Hz sinusoid.
 4. The method of claim 2,wherein said waveform element is constructed from selected phases of athree-phase power supply to have a desired frequency less than 60 Hz. 5.The method of claim 2, wherein said binary code is pseudo-random, saidsource waveform has a predetermined length, said length being sufficientto further reduce said correlation side lobes to a predetermined level,said reference waveform is said source waveform, and said correlation iscircular correlation.
 6. The method of claim 5, wherein said binary codeis a maximal length shift-register sequence.
 7. The method of claim 2,wherein said binary code is a maximal length shift-register sequencewith said resulting source waveform modified such that negative polarityelements in said source waveform are zeroed, said reference waveform issaid source waveform before said negative polarity waveform elements arezeroed, and said correlation is circular correlation.
 8. A method forelectroseismic prospecting of a subterranean formation, said methodcomprising the steps of: (a) constructing a first source waveform and asecond source waveform from a single element, said element consisting ofa single full cycle of a preselected periodic waveform, said periodicwaveform having a frequency predetermined to give desired depthpenetration of said subterranean formation, said elements being piecedtogether with polarities specified sequentially by one member of a Golaycomplementary pair of binary sequences in the case of said first sourcewaveform, and by the second member of said Golay complementary pair inthe case of said second source waveform; (b) generating each of said twosource waveforms as an electrical signal, and transmitting each saidelectrical signal, in turn, into said subterranean formation; (c)detecting and recording seismic signals resulting from conversion ofsaid electrical signals to seismic energy in said subterraneanformation; (d) correlating said recorded seismic signals from each ofsaid source waveforms with said respective source waveform itself; and(e) summing said pair of correlations of said recorded seismic signalsand their corresponding source waveform.to produce a correlated seismicrecord; and (f) creating an image of the subterranean formation from thecorrelated seismic record.
 9. The method of claim 8, wherein saidwaveform element is a single cycle of a 60 Hz sinusoid.
 10. The methodof claim 8, wherein said Golay complementary pair of binary sequencesare selected from other Golay pairs using the criteria of smallestautocorrelation side lobe amplitudes prior to summing.
 11. An electricalsignal for use in electroseismic prospecting of a subterraneanformation, said signal having a waveform constructed from a singleelement, said element consisting of a single full cycle of a preselectedperiodic waveform, said elements being pieced together with polaritiessequentially specified by a preselected binary code, said periodicwaveform having a frequency predetermined to give desired depthpenetration of said subterranean formation, said binary code beingselected to generate side lobe amplitudes below a predetermined levelwhen the signal waveform is correlated with itself.
 12. The electricalsignal of claim 11, wherein said waveform element is a single cycle of a60 Hz sinusoid.
 13. The electrical signal of claim 11, wherein saidwaveform element is constructed from selected phases of a three-phasepower supply to have a desired frequency less than 60 Hz.
 14. Theelectrical signal of claim 11, wherein said binary code is pseudo-randomand said correlation is circular correlation.
 15. The electrical signalof claim 14, wherein said signal waveform has a predetermined length,said length being sufficient to further reduce said side lobe amplitudesto a predetermined level.
 16. The electrical signal of claim 14, whereinsaid binary code is a maximal length shift-register sequence.
 17. Anelectrical signal for use in electroseismic prospecting of asubterranean formation, said signal having a waveform constructed from asingle element, said element consisting of a single full cycle of apreselected periodic waveform, said periodic waveform having a frequencypredetermined to give desired depth penetration of said subterraneanformation, said elements being pieced together with polaritiessequentially specified by a maximal length shift-register sequence, saidresulting signal waveform being modified such that resulting negativepolarity elements are zeroed.
 18. A pair of complementary electricalsignals for use in conjunction with each other in electroseismicprospecting of a subterranean formation, said signals having waveformsconstructed from a single element, said element consisting of a singlefull cycle of a preselected periodic waveform, said periodic waveformhaving a frequency predetermined to give desired depth penetration ofsaid subterranean formation, said elements being pieced together withpolarities sequentially specified by one member of a Golay complementarypair of binary sequences in the case of one of said two electricalsignals, and by the second member of said Golay complementary pair inthe case of the other electrical signal.
 19. The electrical signals ofclaim 18, wherein said waveform element is a single cycle of a 60 Hzsinusoid.
 20. The electrical signals of claim 18, wherein said waveformelement is constructed from selected phases of a three-phase powersupply to have a desired frequency less than 60 Hz.
 21. A method forelectroseismic prospecting of a subterranean formation, said methodcomprising: (a) obtaining a source waveform selected to reduceamplitudes of side lobes produced by correlation with a selectedreference waveform; (b) generating the selected source waveform as anelectrical signal and transmitting it into the subterranean formation;(c) detecting and recording seismic signals resulting from conversion ofthe electrical signal to seismic energy in the subterranean formation;(d) obtaining a correlated seismic record generated by correlating theseismic signals with the reference waveform; and (e) obtaining an imageof the subterranean formation produced from the correlated seismicrecord.
 22. The method of claim 21, wherein said source waveform isconstructed from a single element, said element consisting of a singlefull cycle of a preselected periodic waveform, said elements beingpieced together with polarities sequentially specified by a preselectedbinary code, said periodic waveform having a frequency predetermined togive desired depth penetration of said subterranean formation.
 23. Themethod of claim 22, wherein the waveform element is a single cycle of a60 Hz sinusoid.
 24. The method of claim 22, wherein the waveform elementis constructed from selected phases of a three-phase power supply tohave a desired frequency less than 60 Hz.
 25. The method of claim 22,wherein said binary code is pseudo-random, said source waveform has apredetermined length, said length being sufficient to further reducesaid correlation side lobes to a predetermined level, said referencewaveform is said source waveform, and said correlation is circularcorrelation.
 26. The method of claim 25, wherein said binary code is amaximal length shift-register sequence.
 27. The method of claim 22,wherein said binary code is a maximal length shift-register sequencewith said resulting source waveform modified such that negative polarityelements in said source waveform are zeroed, said reference waveform issaid source waveform before said negative polarity waveform elements arezeroed, and said correlation is circular correlation.
 28. A method forelectroseismic prospecting of a subterranean formation, said methodcomprising: (a) selecting a source waveform and corresponding referencewaveform, said two waveforms being selected to reduce amplitudes of sidelobes produced by correlating said source waveform with said referencewaveform; (b) obtaining recorded seismic signals resulting fromgeneration of said source waveform into an electrical signal andtransmitting it into said subterranean formation; (c) correlating saidrecorded seismic signals with said reference waveform to produce acorrelated seismic record; and (d) creating an image of the subterraneanformation from the correlated seismic record.
 29. The method of claim28, wherein said source waveform is constructed from a single element,said element consisting of a single full cycle of a preselected periodicwaveform, said elements being pieced together with polaritiessequentially specified by a preselected binary code, said periodicwaveform having a frequency predetermined to give desired depthpenetration of said subterranean formation.
 30. The method of claim 29,wherein the waveform element is a single cycle of a 60 Hz sinusoid. 31.The method of claim 29, wherein the waveform element is constructed fromselected phases of a three-phase power supply to have a desiredfrequency less than 60 Hz.
 32. The method of claim 29, wherein saidbinary code is pseudo-random, said source waveform has a predeterminedlength, said length being sufficient to further reduce said correlationside lobes to a predetermined level, said reference waveform is saidsource waveform, and said correlation is circular correlation.
 33. Themethod of claim 32, wherein said binary code is a maximal lengthshift-register sequence.
 34. The method of claim 29, wherein said binarycode is a maximal length shift-register sequence with said resultingsource waveform modified such that negative polarity elements in saidsource waveform are zeroed, said reference waveform is said sourcewaveform before said negative polarity waveform elements are zeroed, andsaid correlation is circular correlation.
 35. A method forelectroseismic prospecting of a subterranean formation, said methodcomprising: (a) obtaining two source waveforms constructed by repeatinga single element, said element consisting of a single full cycle of aperiodic waveform, said periodic waveform having a frequency determinedto give desired depth penetration of the subterranean formation, saidelements being pieced together with polarities specified sequentially byone member of a Golay complementary pair of binary sequences in the caseof one source waveform, and by the second member of the Golaycomplementary pair in the case of the other source waveform; (b)generating each of the two source waveforms as an electrical signal andtransmitting each said electrical signal, in turn, into the subterraneanformation; (c) detecting and recording seismic signals resulting fromconversion of each of the two electrical signals to seismic energy inthe subterranean formation; (d) obtaining a correlated seismic recordgenerated by correlating the seismic signals with the source waveformused to generate them and then summing the correlated record due to onesource waveform with the correlated record due to the other sourcewaveform; and (e) obtaining an image of the subterranean formationproduced from the summed correlated seismic record.
 36. The method ofclaim 35, wherein said waveform element is a single cycle of a 60 Hzsinusoid.
 37. The method of claim 35, wherein said Golay complementarypair of binary sequences are selected from other Golay pairs using thecriteria of smallest autocorrelation side lobe amplitudes prior tosumming.
 38. A method for electroseismic prospecting of a subterraneanformation, said method comprising: (a) constructing a first sourcewaveform and a second source waveform from a single element, saidelement consisting of a single full cycle of a pre-selected periodicwaveform, said periodic waveform having a frequency pre-determined togive desired depth penetration of the subterranean formation, saidelements being pieced together with polarities specified sequentially byone member of a Golay complementary pair of binary sequence in the caseof the first source waveform and by the second member of the Golaycomplementary pair in the case of the second source waveform; (b)obtaining recorded seismic signals resulting from generation of each ofsaid two source waveforms as an electrical signal and transmission ofeach electrical signal, in turn, into the subterranean formation whereeach was converted to seismic energy, (c) correlating said recordedseismic signals from each of the two source waveforms with thecorresponding source waveform itself, thereby producing two correlatedrecords; (d) summing the two correlated records to produce a correlatedseismic record; and (e) creating an image of the subterranean formationfrom the correlated seismic record.
 39. The method of claim 38, whereinsaid waveform element is a single cycle of a 60 Hz sinusoid.
 40. Themethod of claim 38, wherein said Golay complementary pair of binarysequences are selected from other Golay pairs using the criteria ofsmallest autocorrelation side lobe amplitudes prior to summing.